The present invention relates to liquid-vapor contact trays for use in divided wall exchange columns for heat and/or mass transfer processes. The invention has particular application in cryogenic air separation processes utilizing distillation, although it also may be used in other heat and/or mass transfer processes which use such trays.
As used herein, the term “column” (or “exchange column”) means a distillation or fractionation column or zone, i.e., a column or zone where liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
A divided wall column is in principle a simplification of a system of thermally coupled distillation columns. In divided wall columns, a dividing wall is located in the interior space of the column. The dividing wall generally is vertical. Two different mass transfer separations may occur on either side of the dividing wall.
The exchange column usually contains some form of vapor-liquid contacting device which may be in the form of packing, such as random or structured packing, or in the form of a fractionation tray(s). A fractionation tray typically comprises a large flat area referred to as the decking or contacting deck of the tray plus means to deliver liquid to the tray from the next tray above and to remove liquid for passage to the next tray below.
The liquid being removed from the tray flows through a part of the tray referred to as a downcomer. The downcomer is a conduit for downward passing of liquid arranged in an opening in a panel of the contacting deck. In some downcomers, a part of the wall of the downcomer may extend above the panel and is called the outlet weir, and the part of the downcomer extending below the panel is called the downcomer apron. However, outlet weirs often are separate mechanical pieces and not necessarily an extension of the downcomer wall. In fact, trays may be designed without outlet weirs.
Vapor generated in the lower portion of the column passes upward through perforations in the decking, while the liquid flows downward from tray to tray countercurrent to the vapor. With a “crossflow tray” the liquid first enters upon a tray from the downcomer of the tray above. It then passes across the decking of the tray and finally exits through an outlet downcomer of the tray. U.S. Pat. No. 6,645,350 (Steacy) discloses one type of “crossflow tray.”
During normal operation, liquid collected on a tray flows over a perforated panel of the deck where it is contacted with upwardly flowing vapor passing through the perforations. Then the liquid flows over the outlet weir into the downcomer and onto the receiving area of the perforated panel of the tray below, and so on. The downcomers of two adjacent trays are not placed directly above each other, but spaced apart in lateral direction (or staggered) in order to prevent liquid from falling directly into the downcomer of the lower tray.
One type of prior art tray uses a “racetrack” tray design, which has liquid flowing around an annular area with downcomers also placed radially. For example, U.S. Pat. No. 1,950,313 (Linde) discloses a “racetrack” tray.
It is desired to have an improved tray or trays for use in a divided wall exchange column which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is further desired to have an improved cryogenic air separation plant having an improved tray or trays for use in a divided wall exchange column which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.